Loading of hydrophobic drugs into hydrophilic polymer delivery systems

ABSTRACT

A process is described for loading hydrophilic polymer particles with a water-insoluble solvent-soluble drug. The particles are preferably embolic agents. The method provides particles having little or no drug at the surface and in a surface layer, whereby the burst effect is minimised. The drug is precipitated in the core of the particle, leading to extended release. The drug is, for instance, paclitaxel, rapamycin, dexamethasone or ibuprofen.

The present invention relates to methods for preparing hydrophobic drugloaded hydrophilic microspheres, having a non-burst and sustainedrelease local delivery of drug at the site of embolisation.

Embolisation therapy involves the introduction of an agent into thevasculature in order to bring about the deliberate blockage of aparticular vessel. This type of therapy is particularly useful forblocking abnormal connections between arteries and veins (such asarteriovenous malformations, or AVMs), and also for occluding vesselsthat feed certain hyper-vascularised tumours, in order to starve theabnormal tissue and bring about tumour ischemia or necrosis.

The process of embolisation may induce tumour necrosis or ischemiadepending upon the extent of the embolisation. The response of thetumour cells to the hypoxic environment can result in an ensuingangiogenesis in which new blood vessels are grown to compensate for theloss of flow to the tumour by the embolisation. It would be desirabletherefore to combine embolisation with the administration of agents thatcould prevent the ensuing angiogenic response or combine with therelease of a cytotoxic or other anti-tumoral agent to bring about celldeath in those cells that are not killed by the embolisation.

In the early 1960s, the National Cancer Institute (NCI) in the UnitedStates initiated a programme of biological screening of extracts takenfrom a wide variety of natural sources. One of these extracts was foundto exhibit marked antitumour activity against a broad range of rodenttumours. Although this discovery was made in 1962, it was not until fiveyears later that two researchers, Wall and Wani, of the ResearchTriangle Institute, North Carolina, isolated the active compound, fromthe bark of the Pacific yew tree (Taxus brevifolia). In 1971, Wall andWani published the structure of this promising new anti-cancer leadcompound, a complex poly-oxygenated, Wani, M. C., H. L. Taylor, MonroeWall, P. Coggon, A. T. McPhail, 1971, “Plant Antitumor Agents. VI. TheIsolation and Structure of Taxol, a Novel Antileukemic and AntitumorAgent from Taxus brevifolia,” Journal of the American Chemical Society,93: 2325-2327.

Paclitaxel is a natural product with antitumor activity. It is used totreat ovarian cancer, Karposi's sarcoma, and used in combinations withother chemotherapy agents to treat breast cancer, non-small cell lungcancer and is most effective against ovarian carcinomas and advancedbreast carcinomas. Paclitaxel is given intravenously (it irritates skinand mucous membranes on contact). Paclitaxel, which is sold as Taxol® byBristol-Myers Squibb, is obtained via a semi-synthetic process fromTaxus baccata. The chemical name is5β,20-Epoxy-1,2α,4,7β,10β,13α-hexahydroxytax-11-en-9-one 4,10-diacetate2-benzoate 13-ester with (2 R,3S)-N-benzoyl-3-phenylisoserine.Paclitaxel is a white to off-white crystalline powder with the empiricalformula CH₄₇H₅₁NO₁₄ and a molecular weight of 853.9. Paclitaxel ishighly lipophilic, insoluble in water, and melts at around 216-217° C.

The relatively non-toxic properties of paclitaxel have made it a leadinglight in the treatment of cancer in the 1990s, providing a non-intrusivealternative to the more radical techniques of radiotherapy and surgery.

Despite its well-documented biological activity, very little interestwas shown in paclitaxel until scientists at the Albert Einstein MedicalCollege reported that its mode of action was totally unique. Until thisfinding in 1980, it was believed that the cytotoxic properties ofpaclitaxel were due to its ability to destabilise microtubules,important structures involved in cell division (mitosis). In fact,paclitaxel was found to induce the assembly of tubulin intomicrotubules, and more importantly, that the drug actually stabilisesthem to the extent that mitosis is disrupted. Such a novel mode ofaction was believed to make paclitaxel a prototype for a new class ofanticancer drugs. Paclitaxel binds to microtubules and inhibits theirdepolymerization (molecular disassembly) into tubulin. It blocks acell's ability to break down the mitotic spindle during mitosis (celldivision). With the spindle still in place the cell cannot divide intodaughter cells (this is in contrast to drugs like colchicine and theVinca alkaloids, which block mitosis by keeping the spindle from beingformed in the first place).

Most of the reported work on the preparation of paclitaxel-loadedpolymeric drug delivery systems is based on hydrophobic polymer systemsin which paclitaxel has good solubility.

WO2003/077967 relates to a deposition method for applying an activesubstance to an endoprosthesis having a thin polymer coating. Thedeposition method enables a slow and largely constant administering ofan active substance, as cited in the example of tretinoin. Sinceadditional processing steps are not required after the application ofthe active substance(s), it is unnecessary to worry about coatingconditions causing the active substance to be broken down, for example,by the application of second polymer coating. Even relatively unstableactive substances, e.g. tretinoin, can be applied without anydifficulties to the endoprosthesis. Thus 4-amino-[2,2]-paracyclophanewas cleaved at 700° C., 20 Pa to reactive monomers and polymerised atthe surface of a stent at 20° C. The polymer-coated stent was contactedwith a DMSO solution of tretinoin and dipped into water; this resultedthe precipitation of tretinoin onto the surface of the stent andembedding of the precipitate into the polymer layer.

Angiotech's group have studied the paclitaxel loading into poly(L-lacticacid) (PLLA) microspheres using solvent evaporation method. PLLA andpaclitaxel were dissolved in dichloromethane. The organic phase wasadded to an aqueous solution of 2.5% poly (vinyl alcohol) understirring. Subsequently, after 2 hr the aqueous suspension containingmicrospheres was passed through sieves to retain the particles incertain size ranges. The microspheres were further dried for 12-16 hr atambient temperature. [Richard T. Liggins, Helen M. Burt ‘Paclitaxelloaded poly(L-lactic acid microspheres: properties of microspheres madewith low molecular weight polymers’ International Journal ofPharmaceutics 222 (2001) 19-33; Richard T. Liggins, Helen M. Burt‘Paclitaxel loaded poly(L-lactic acid) microspheres II. The effect ofprocessing parameters on microsphere morphology and drug releasekinetics’ International Journal of Pharmaceutics 281 (2004) 103-106.]Later they extended their work on poly(lactic-co-glycolic acid) filmsfor delivery of paclitaxel. [John K. Jackson, et al. ‘Characterizationof perivascular poly(lactic-co-glycolic acid] films containingpaclitaxel’ International Journal of Pharmaceutics 283 (2004) 97-109.]Other work includes PEG-coated poly(lactic acid) microspheres. [GladwinS. Das, et al. ‘Controlled delivery of taxol from poly(ethyleneglycol)-coated poly(lactic acid) microspheres’ Journal of BiomedicalMaterials Research 55 (2001) 96-103].

Boston Scientific Corporation has developed the system of coronary stentcoating for delivery of paclitaxel by formulating polymer blends with 10to 25% of paclitaxel. The polymers used are poly(butyl methacrylate),poly(styrene-co-isobutylene-co-styrene), orpoly(styrene-co-(ethylene-butylene)-co-styrene), which are blended withpoly(styrene-co-maleic anhydride). A recent development uses a modifiedstyrenic portion, i.e. hydroxystyrene or its acetylated version.[Shrirang Ranade, et al. Abstracts of papers, 229th ACS nationalMeeting, San Diego, Calif., US, Mar. 13-17, 2005, PMSE-022].

Composition and methods for in vivo controlled release ofpharmaceutically active agents associated with hydroxyapatite (HAP) in apharmaceutically acceptable carrier are described in WO2003030943. Thepharmaceutically acceptable carrier can be a polymer paste or gel whichmay contain a second pharmacologically active agent. Methods of makingand administering controlled release compositions for the delivery of apharmacologically active agent, such as a nucleic acid, in combinationwith a polycationic polymer and in a pharmaceutically acceptablecarrier, to a mammal in a pharmaceutically effective amount areprovided.

Rapamycin, also known as sirolimus, was isolated the first time in 1969from a fungus (Streptomyces hygroscopicus) in the island of Rapa Nui(Easter Island). Initially it was found to have potent antifungal andantiproliferative activities; but it was in 1977 when Martel et alreported its promising immunosuppressive activity [Martel, R. R.;Canadian Journal of Physiological Pharmacology, 55, 48-51 (1977).] Fromthis time its mechanism of action has been thoroughly studied, and it isknown how this antibiotic exerts its immunosuppressive andantiproliferative activities. Rapamycin is a white to off-white powderand is insoluble in water, but freely soluble in benzyl alcohol,chloroform, acetone, and acetonitrile.

Rapamycin and rapamycin analogues are currently in clinical developmentagainst a number of cancer indications. The mechanism of action is as aninhibitor of the mammalian target of rapamycin (mTOR). The cyclicmacrolide structure inhibits cellular proliferation by interfering withthe highly conserved TOR pathway, which control the synthesis ofessential proteins involved in cell cycle progression.

mTOR is a protein kinase with similarities to the catalytic domains ofphosphoinositide 3-kinases (PI3-k). Once activated, TOR transducessignals that initiate synthesis of ribosomal proteins, translation ofspecific mRNAs and generation of cyclin-dependent kinases, promoting theprogression of the cell cycle. This results in activation andproliferation of T and B-cells and antibody production as well asproliferation of non-immune cells such as hepatocytes, fibroblasts,endothelial cells and smooth muscle cells. [Neuhaus P, Klupp J, LangrehrJ M.; Liver Transpl. 2001 June; 7(6):473-84. mTOR inhibitors: anoverview.]

Rapamycin exerts its antiproliferative effect mainly by blocking all ofthese events, as a consequence of inhibition of mTOR. It is able toinhibit this protein kinase by forming a trimeric stable complex, afterbinding with the soluble intracellular receptor protein FKBP12. Thisinhibition blocks the synthesis of cyclin-dependent kinases, which arekey mRNAs that code for proteins required for cell cycle progressionfrom G1 to S phase.

mTOR is also a positive regulator of hypoxia-induciblefactor-1-dependent gene transcription in cells exposed to hypoxia orhypoxia mimetic agents [Hudson C C, Liu M, Chiang G G, Otterness D M,Loomis D C, Kaper F, Giaccia A J, Abraham R T.; Mol Cell Biol. 2002October ; 22(20):7004-14. Regulation of hypoxia-inducible factor 1alphaexpression and function by the mammalian target of rapamycin.] Ifrapamycins prove to be effective inhibitors of hypoxic adaptation indeveloping tumours, these drugs could have dramatic effects on tumourgrowth, invasiveness and metastatic potential in cancer patients. Inembolisation a hypoxic environment is induced and therefore rapamycinand its analogues may act mechanistically by inhibiting mTOR andconsequently inhibiting the production of hypoxia induced factor (HIF-1)widely believed to be involved in angiogenic responses.

Treatment of tumour-bearing animals with rapamycin results in decreasedexpression of VEGF mRNA and decreased circulating levels of VEGFprotein. Thus, proliferation of smooth muscle and endothelial cells isinhibited by mTOR inhibition. This anti-angiogenic effect may contributeto the efficacy of mTOR inhibitors in cancer therapy [Rao R D, Buckner JC, Sarkaria J N.; Curr Cancer Drug Targets. 2004 December; 4(8):621-35.Mammalian target of rapamycin (mTOR) inhibitors as anti-cancer agents].

Rapamycin and rapamycin analogues have demonstrated activity against abroad range of human cancers growing in tissue culture and in humantumor xenograft models. The central role of mTOR in modulating cellproliferation in both tumour and normal cells and the importance of mTORsignalling for the hypoxic response suggest that rapamycin-basedtherapies may exert anti-tumour effects primarily through eitherinhibition of tumour cell proliferation or suppression of angiogenesis.Although rapamycin can induce apoptosis in select tumour models,rapamycin treatment typically slows growth but does not induce tumourregression, suggesting that tumour cell loss through apoptosis or othermechanisms are not major contributors to drug effect in most cases.

There have been many reports of drug delivery systems using hydrophobicpolymers, such as poly(L-lactic acid), poly(lactic-co-glycolic acid),poly(caprolactone), polybutyl methacrylate, andpoly(styrene-co-isobutylene-co-styrene). However, there are few reportsof hydrogel microspheres loaded with paclitaxel. This is due to the poorcompatibility between hydrophobic drugs and hydrogel microspheres [R.Shi, H. M. Burt, ‘Amphiphilic dextran-graft-poly(epsilon-caprolactone)films for the controlled release of paclitaxel’ International Journal ofPharmaceutics 271 (2004) 167,http://www.ptca.org/articles/taxus_profileframe.html D. S. Das, G. H. R.Rao, R. F. Wilson, T. Chandy, ‘Controlled delivery of taxol frompoly(ethylene glycol)-coated poly(lactic acid) microspheres’ Journal ofBiomedical Materials Research, 55 (2001) 96 R. T. Liggins, H. M. Burt,‘Paclitaxel loaded poly(L-lactic acid) microspheres: properties ofmicrospheres made with low molecular weight polymers’ InternationalJournal of Pharmaceutics, 222 (2001) 19. J. K. Jackson, J. Smith, K.Letchford, K. A. Babiuk, L. Machan, P. Signore, W. L. Hunter, K. Wang,H. M. Burt, ‘Characterisation of perivascular poly(lactic-co-glycolicacid) films containing paclitaxel’ International Journal ofPharmaceutics, 283 (2004) 97. S. K. Dordunoo, J. K. Jackson, L. A.Arsenault, A. M. C. Oktaba, W. L. Hunter, H. M. Burt, ‘Taxolencapsulation in poly(epsilon-caprolactone) microspheres’ Cancerchemother. Parmacol. 36 (1995) 279.]

US2003/202936 discloses a process in which microspheres are prepared byimmersing microparticles in a solution containing methanol andaminoacridine. Excess methanol is removed by evaporation, but thisresults in precipitation of the aminoacridine both inside and outsidethe microspheres.

Vandelli et al in the Journal of Controlled Release, 96(2004), 67-84disclose microspheres in which diclofenac is precipitated in the core.The drug is uniformly distributed in each microparticle. The presence ofdrug on or close to the surface leads to rapid initial release of thedrug, which is often undesirable.

According to the present invention there is provided a new process forforming drug-loaded polymer particles comprising the steps:

a) contacting particles comprising a matrix of water-insoluble polymer,which particles, when neat, are swellable in phosphate buffered saline(PBS) at room temperature to an equilibrium water content in the rangeof from 40% to 99% by weight based on polymer plus PBS, with a solutionof a drug having a water solubility of less than 10 g/l at roomtemperature, in a first organic solvent; whereby a solution of drug insolvent becomes impregnated into the particles and the first solvent isselected to be capable of swelling neat particles;

b) separating drug solution which has not impregnated the particles instep a) from the impregnated particles;

c) contacting the impregnated particles with aqueous liquid whereby drugis precipitated in the core of the particles; and further comprising thesteps d) and/or e)

d) rinsing the particles with drug precipitated within the core with avolatile, second solvent in which the particles are less swellable,relative to their swellability in water, and which is a solvent for thedrug, wherein the drug solubility in the second solvent is at least 0.1g/l, whereby drug on and close to the surface of the particles isremoved with the second solvent;

e) drying the drug-loaded polymer particles by vacuum, or freeze drying,or air flow to remove the second solvent.

By “neat particles” we mean particles which are not impregnated withsolvent, such as particles which have been dried by, for instance,lyophilisation or solvent drying.

When the solution of drug in a first solvent impregnates the particles,the solution mixes with any liquid which is already impregnated into theparticles. The particles may either swell, or shrink. It is importantthat the drug remains in solution when the particles become impregnatedwith the drug solution.

Generally, the particles have a water content of less than 10% based onthe weight of polymer matrix. This helps to ensure that the drug remainsin solution, when the particles become impregnated.

Generally the particles are supplied at least partially swollen withaqueous impregnant liquor e.g. having at least 40% by weight waterimpregnated into the particles, based on the weight of polymer pluswater. Since step a) requires the drug to remain in solution when theparticles are impregnated, water-swollen particles must be subjected topreliminary steps to remove water. Although evaporation may be used toremove the water, it is more convenient to mix the particles, even inthe presence of excess impregnant water, with a water-miscible solventto swell the particles and replace absorbed water by solvent. Theextracted water is removed from swollen particles as a liquid mixturewith the solvent. Addition of further aliquots of the solvent is thenmade with removal of solvent/water mixtures, until the level of water istypically less than 10% based on the weight of polymer matrix. Thisprocedure is referred to as prewashing hereinafter. The prewashingsolvent is conveniently the same as the first solvent. The level ofwater remaining in the particles following the prewash, for instancewhen saturated with prewash solvent, calculated from the weight ofsolvent added to each step, the weight of water swollen into the matrix,the weight of mixed solvent and water removed in each step and thenumber of steps, assuming complete mixing and dilution of absorbedliquids as well as of non-absorbed liquids.

In the invention the first and second, volatile organic solvents areselected having regard to their ability to dissolve the drug and changethe drug-loading capacity of the polymer particles. The first solvent isselected to be capable of swelling neat particles. The volatile solventis mainly for the purpose of cleaning the polymer particle surface andpreferably to extract water left in step c). The first and second(volatile) solvent may be the same as one another or may be different.

Preferably, the second (volatile) solvent has a boiling point of lessthan 90° C.

Preferably the first and/or second (volatile) solvents are those thatswell the beads and can be water-miscible or water-immiscible. Howeverin a less desirable form of the invention, solvents that shrink thebeads can be used. Useful solvents include polar aprotic solvents suchas dimethylsulphoxide (DMSO), a lactone, for example a pyrrolidone, suchas 1-methyl-2-pyrrolidinone (NMP), dialkyl formamide, for instancedimethyl formamide (DMF), or a cyclic ether, for instance 1,4-dioxane(“dioxane”), but is preferably DMSO. The solvent may be a proticsolvent, such as an alcohol.

The present invention has been found to be of utility for formulatingdrugs having anti-tumour properties and low solubility in water, withhigher solubility in a water-miscible organic solvent. The invention isof particular utility for, for instance paclitaxel and derivativeshaving solubility in water at room temperature less than 10 g/l,rapamycin and derivatives having solubility in water at room temperatureless than 10 g/l, dexamethasone and derivatives having solubility inwater at room temperature less than 10 g/l, methotrexate, and sometecans with water-solubility less than 10 g/l. All of these compoundshave a solubility ratio in a water-miscible solvent to water at roomtemperature of at least 10:1, preferably at least 100:1, up to as muchas 10⁶:1 or even more, for instance more than 10³:1.

For these compounds the following table gives comparison water andsolvent solubilities at room temperature. The ratio is of solubility insolvent:solubility in water (insoluble means less than 10 mg/l).

Drug Sol^(y) in water mg/l Sol^(y) in DMSO Sol^(y) Ratio Paclitaxel0.3-30 50 >1.7 × 10³   Methotrexate insol 200 >5 × 10⁴ Rapamycin 0.6925 >4 × 10⁴ Dexamethasone insol 600 >10⁵ Camptothecin insol 10 >10³

After prewashing, the polymer particles are impregnated with drugsolution, and contact with the solution takes place for sufficient timefor the particles to be loaded to equilibrium. Preferably, the particlesare swollen to equilibrium. Alternatively the particles may be partiallyswollen, for instance to a solvent concentration of at least 50%equilibrium, more preferably at least 75% equilibrium concentration atroom temperature. The degree of swelling may be monitored using amicroscope. Swelling to equilibrium is reached when there is no furtherincrease in average size (or volume) of the particle.

In the precipitation step (step c)), aqueous liquor is contacted withthe solvent-laden particles for sufficient time to allow diffusion ofwater into the core of the particles and precipitation of drug to takeplace throughout the particles. Since the drug is water-insoluble, theuse of excess aqueous liquor at this stage should lead to little by wayof drug loss. Instead drug is immobilised by precipitating within thepolymer matrix whereby it is immobilised.

Contact with aqueous liquor is generally carried out at a temperature<25° C. for a period of at least around 1 minute, preferably withagitation to optimise water/particle contact.

In the rinsing step, the solvent is contacted with the swollen particlesfor sufficient time to create a drug free layer on the particle surface.Selection of a suitable solvent for this step may involve a screeningprocess in which water-swollen, but non-drug containing polymerparticles are contacted with the solvent for varying periods of time,with the particles being observed before and after the solvent contact.Observation may be under a light microscope, optionally with measurementof the particle diameter and shape. Since the solvent could partiallyde-swell the polymer, particle size after the contact generally will belower. The surfaces of the particles may also be observed to be lesssmooth, with angularities, or wrinkles.

The solvent for the rinsing step should also be selected such that thedrug is at least slightly soluble in the solvent. The solubility shouldbe at least 1 g/l. The rinsing step results in drug precipitate within asurface layer of the particles being dissolved and removed with rinsingsolvent, to leave a relatively drug-free surface layer of polymer. Thissurface layer is dependent on the particle diameter and is generallyaround 1 to 100 μm thick, for instance about 30 μm thick. The thicknessof the surface layer may be observed by placing the particles under anoptical microscope. The polymer is substantially transparent, whereasthe precipitated drug in the core of the particles renders this portiontranslucent or opaque. The particles therefore have a translucent oropaque core with a transparent halo of surface layer surrounding thecore. The polymer, however, may be analysed and shown to comprise achemically homogeneous material extending from the core to the outersurface of the particles, with the surface layer differing from the corematerial by the absence of drug.

According to the present invention there is also provided drug loadedpolymer particles having a homogeneous polymer composition from thecentre to the periphery, having precipitated drug in a core regionthereof, which drug has a water solubility of less than 0.1 g/l at roomtemperature and having a surface layer in the range of from 1 to 100 μmthick, wherein the ratio of the concentration of drug in thecore:concentration of drug in the surface layer is at least 2:1,preferably at least 10:1, more preferably at least 100:1.

Preferably, the drug has a solubility in a solvent selected fromdimethyl sulfoxide (DMSO), 1-methyl-2-pyrrolidinone (NMP), dimethylformamide (DMF) and dioxane to a concentration of at least 10¹,preferably at least 10², times the solubility in water at roomtemperature.

The polymer which is used to form the particles should be a relativelyhydrophilic polymer that must be water-insoluble. By water-insoluble wemean that the polymer will not dissolve in water, or may be swollen bywater, but constrained from total dissolution by physical or chemicalcrosslinks. The polymer thus forms the hydrogel on contact with water. Ahydrogel may comprise, for instance, at least 40%, preferably at least60%, more preferably at least 75%, preferably more than 80%, morepreferably more than 90% and most preferably at least 95% water when theparticles are swollen to equilibrium in PBS at room temperature. Theequilibrium water content after swelling to equilibrium may be tested bygravimetric methods.

The beads before loading with drug have a diameter substantially all inthe range 25 to 1500 μm, preferably in the range 50 to 1200 μm, forinstance in the range 100 to 1200 μm measured in PBS at room temperatureby optical microscopy.

In the invention the term bead is intended to cover particles of allshapes, for instance rod shapes, cubes, irregular and non-uniformshapes. However the invention is of most benefit where the beads arespherical, spheroidal or pellet shaped, or disk shaped. In non-sphericalparticles, such as pellets, spheroids or disks, the maximum dimension ispreferably no more than three times the minimum diameter, and preferablyless than two times the minimum diameter, for instance around 1.5 orless. The size limitations mentioned above are determined by testing asample of the swellable beads under conditions in which the beads areswollen to equilibrium in phosphate buffered saline at room temperatureand the sizes are measured using an optical microscope.

The compositions are preferably provided with a particle sizespecification which defines the spread of diameters. Preferably thebeads are graded into calibrated size ranges for accurate embolisationof vessels. The particles preferably have sizes when equilibrated in PBSat room temperature, in the range 100 to 1500 μm, more preferably in therange 100 to 1200 μm. The calibrated ranges may comprise beads havingdiameters with a nominal bandwidth of about 100 to 300 μm. The nominalsize ranges may be for instance 100 to 300 μm, 300 to 500 μm, 500 to 700μm, 700 to 900 μm and 900 to 1200 μm.

Preferably the polymer comprises alcoholic hydroxyl groups or acylatedderivatives thereof. In one embodiment polymers are used which arederived from natural sources, such as albumin, alginate, gelatin,starch, chitosan or collagen, all of which have been used as embolicagents. In a preferred embodiment the polymer is substantially free ofnaturally occurring polymer or derivatives. It is preferably formed bypolymerising ethylenically unsaturated monomers including monomershaving hydroxyalkyl or acyloxyalkyl groups in the presence of di- orhigher-functional crosslinking monomers. The ethylenically unsaturatedmonomers may include an ionic (including zwitterionic) monomer.

Copolymers of hydroxyethyl methacrylate, acrylic acid and cross-linkingmonomer, such as ethylene glycol dimethacrylate or methylenebisacrylamide, as used for etafilcon A based contact lenses may be used.Copolymers of N-acryloyl-2-amino-2-hydroxymethyl-propane-1,3-diol andN,N-bisacrylamide may also be used.

Other polymers are cross-linking styrenic polymers e.g. with ionicsubstituents, of the type used as separation media or as ion exchangemedia.

Another type of polymer which may be used to form the water-swellablewater-insoluble matrix is polyvinyl alcohol crosslinked usingaldehyde-type crosslinking agents such as glutaraldehyde. For suchproducts, the polyvinyl alcohol (PVA) may be rendered ionic by providingpendant ionic groups by reacting a functional ionic group containingcompound with the hydroxyl groups. Examples of suitable functionalgroups for reaction with the hydroxyl groups are acylating agents, suchas carboxylic acids or derivatives thereof, or other acidic groups whichmay form esters.

The invention is of particular value where the polymer matrix is formedfrom a polyvinyl alcohol macromer, having more than one ethylenicallyunsaturated pendant group per molecule, by radical polymerisation of theethylenic groups. Preferably the PVA macromer is copolymerised withethylenically unsaturated monomers for instance including a nonionicand/or ionic monomer including anionic monomer.

The PVA macromer may be formed, for instance, by providing PVA polymer,of a suitable molecular weight such as in the range 1000 to 500,000 D,preferably 10,000 to 100,000 D, with pendant vinylic or acrylic groups.Pendant acrylic groups may be provided, for instance, by reactingacrylic or methacrylic acid with PVA to form ester linkages through someof the hydroxyl groups. Other methods for attaching vinylic groupscapable of polymerisation onto polyvinyl alcohol are described in, forinstance, U.S. Pat. No. 4,978,713 and, preferably, U.S. Pat. Nos.5,508,317 and 5,583,163. Thus the preferred macromer comprises abackbone of polyvinyl alcohol to which is linked, via a cyclic acetallinkage, an (alk)acrylaminoalkyl moiety. Example 1 describes thesynthesis of an example of such a macromer known by the approved namednelfilcon B. Preferably the PVA macromers have about 2 to 20 pendantethylenic groups per molecule, for instance 5 to 10.

Where PVA macromers are copolymerised with ethylenically unsaturatedmonomers including an ionic monomer, the ionic monomer preferably hasthe general formula I

Y¹BQ¹   I

in which Y¹ is selected from

CH₂═C(R¹⁰)—CH₂—O—, CH₂═C(R¹⁰)—CH₂ OC(O)—, CH₂═C(R¹⁰)OC(O)—,CH₂═C(R¹⁰)—O—, CH₂═C(R¹⁰)CH₂OC(O)N(R¹¹)—, R¹²OOCCR¹⁰═CR¹⁰C(O)—O—,R¹⁰CH═CHC(O)O—, R¹⁰CH═C(COOR¹²)CH₂—C(O)—O—,

wherein:

R¹⁰ is hydrogen or a C₁-C₄ alkyl group;

R¹¹ is hydrogen or a C₁-C₄ alkyl group;

R¹² is hydrogen or a C₁-C₄ alkyl group or BQ¹ where B and Q¹ are asdefined below;

A¹ is —O— or —NR¹¹—;

K¹ is a group —(CH₂)_(r)OC(O)—, —(CH₂)_(r)C(O)O—, —(CH₂)_(r)OC(O)O—,—(CH₂)_(r)NR¹³—, —(CH₂)_(r)NR¹³C(O)—, —(CH₂)_(r)C(O)NR¹³—,—(CH₂)_(r)NR¹³C(O)O—, —(CH₂)_(r)OC(O)NR¹³—, —(CH₂)_(r)NR¹³C(O)NR¹³— (inwhich the groups R¹³ are the same or different), —(CH₂)_(r)O—,—(CH₂)_(r)SO₃—, or, optionally in combination with B, a valence bond andr is from 1 to 12 and R¹³ is hydrogen or a C₁-C₄ alkyl group;

B is a straight or branched alkanediyl, oxaalkylene,alkanediyloxaalkanediyl, or alkanediyloligo(oxaalkanediyl) chainoptionally containing one or more fluorine atoms up to and includingperfluorinated chains or, if Q¹ or Y¹ contains a terminal carbon atombonded to B a valence bond; and

Q¹ is an ionic group.

Such a compound including an anionic group Q¹ is preferably included.

An anionic group Q¹ may be, for instance, a carboxylate, carbonate,sulphonate, sulphate, nitrate, phosphonate or phosphate group. Themonomer may be polymerised as the free acid or in salt form. Preferablythe pK_(a) of the conjugate acid is less than 5.

A suitable cationic group Q¹ is preferably a group N⁺R¹⁴ ₃, P⁺R¹⁵ ₃ orS⁺R¹⁵ ₂ in which the groups R¹⁴ are the same or different and are eachhydrogen, C₁₋₄-alkyl or aryl (preferably phenyl) or two of the groupsR¹⁴ together with the heteroatom to which they are attached from asaturated or unsaturated heterocyclic ring containing from 5 to 7 atomsthe groups R¹⁵ are each OR¹⁴ or R¹⁴. Preferably the cationic group ispermanently cationic, that is each R¹⁴ is other than hydrogen.Preferably a cationic group Q is N⁺R¹⁴ ₃ in which each R¹⁴ isC₁₋₄-alkyl, preferably methyl.

A zwitterionic group Q¹ may have an overall charge, for instance byhaving a divalent centre of anionic charge and monovalent centre ofcationic charge or vice-versa or by having two centres of cationiccharge and one centre of anionic charge or vice-versa. Preferably,however, the zwitterion has no overall charge and most preferably has acentre of monovalent cationic charge and a centre of monovalent anioniccharge.

Examples of zwitterionic groups which may be used as Q in the presentinvention are disclosed in WO-A-0029481.

Where the ethylenically unsaturated monomer includes zwitterionicmonomer, for instance, this may increase the hydrophilicity, lubricity,biocompatibility and/or haemocompatibility of the particles. Suitablezwitterionic monomers are described in our earlier publicationsWO-A-9207885, WO-A-9416748, WO-A-9416749 and WO-A-9520407. Preferably azwitterionic monomer is 2-methacryloyloxy-2′-trimethylammonium ethylphosphate inner salt (MPC).

In the monomer of general formula I preferably Y¹ is a groupCH²═CR¹⁰COA- in which R¹⁰ is H or methyl, preferably methyl, and inwhich A¹ is preferably NH. B is preferably an alkanediyl group of 1 to12, preferably 2 to 6 carbon atoms. Such monomers are acrylic monomers.

There may be included in the ethylenically unsaturated monomer diluentmonomer, for instance non-ionic monomer. Such a monomer may be useful tocontrol the pK_(a) of the acid groups, to control the hydrophilicity orhydrophobicity of the product, to provide hydrophobic regions in thepolymer, or merely to act as inert diluent. Examples of non-ionicdiluent monomer are, for instance, alkyl (alk) acrylates and (alk)acrylamides, especially such compounds having alkyl groups with 1 to 12carbon atoms, hydroxy, and di-hydroxy-substituted alkyl(alk) acrylatesand -(alk) acrylamides, vinyl lactams, styrene and other aromaticmonomers.

In the polymer matrix, the level of anion is preferably in the range 0.1to 10 meq g⁻¹, preferably at least 1.0 meq g⁻¹. Preferred anions arederived from strong acids, such as sulphates, sulphonates, phosphatesand phosphonates.

Where PVA macromer is copolymerised with other ethylenically unsaturatedmonomers, the weight ratio of PVA macromer to other monomer ispreferably in the range of 50:1 to 1:5, more preferably in the range20:1 to 1:2. In the ethylenically unsaturated monomer the anionicmonomer is preferably present in an amount in the range 10 to 100 mole%, preferably at least 25 mole %.

The crosslinked polymer may be formed as such in particulate form, forinstance by polymerising in droplets of monomer in a dispersed phase ina continuous immiscible carrier. Examples of suitable water-in-oilpolymerisations to produce particles having the desired size, whenswollen, are known. For instance U.S. Pat. No. 4,224,427 describesprocesses for forming uniform spherical beads (microspheres) of up to 5mm in diameter, by dispersing water-soluble monomers into a continuoussolvent phase, in a presence of suspending agents. Stabilisers andsurfactants may be present to provide control over the size of thedispersed phase particles. After polymerisation, the crosslinkedmicrospheres are recovered by known means, and washed and optionallysterilised. Preferably the particles, e.g. microspheres, are swollen inan aqueous liquid, and classified according to their size.

In the invention the steps in which the particles are contacted withliquid are generally conducted in the presence of excess liquid, forinstance in a vessel with agitation. Alternative methods to contactingthe particles with the liquid are: immersion without agitation,immersion with sonication, continuous flow and fluidised bed. Liquid andloaded particles are generally separated from one another, for instanceby one or a combination of the following methods: pipetting,decantation, filtration, evaporation, liquid exchange, aspiration orlyophilisation. If required, the particles may, at the end of theprocess, be dried, for instance by lyophilisation or solvent drying andmay then be sterilised by autoclaving or gamma irradiation.

The following is a brief description of the figures:

FIG. 1 is a flow chart showing the process steps for Example 1 andothers;

FIG. 2 shows the loading efficacy of paclitaxel against target loadingfor Example 1;

FIG. 3 shows microscopy pictures of dried beads and rehydrated beads asdescribed in Example 1; FIG. 3A shows dried beads with 7.2mg paclitaxelloading, 3B shows rehydrated beads of 3A, 3C shows dried beads with 23.9mg/ml paclitaxel loading and 3D shows rehydrated beads of 3C;

FIG. 4 shows the elution profile for paclitaxel from product formed inExample 1;

FIG. 5 shows a flow chart for the preparation process used in Example 2;

FIG. 6 shows rapamycin loading efficacy in PVA beads using the methoddescribed in Example 2;

FIGS. 7A to D are photographs showing rapamycin loaded beads using themethod of Example 2. FIG. 7A shows 300 to 500 μm unloaded bead, FIG. 7Bshows 300 to 500 μm samples of the bead loaded with 5.1 mg/ml rapamycin,FIG. 7C shows the 300 to 500 μm beads loaded with 8.5 mg/ml rapamycinand FIG. 7D shows the beads loaded with 23.8 mg/ml rapamycin, all asdescribed in Example 2;

FIG. 8 shows the size distribution of rapamycin beads as produced inExample 2;

FIG. 9 shows the elution profile of the beads with the three loadinglevels in Example 2;

FIG. 10 shows certain micrographs of dexamethasone loaded beads asdescribed in Example 3.

The invention is illustrated in the following Examples:

EXAMPLE 1

Preparation of paclitaxel loaded microspheres is as described inWO2004/000548 Example 1, “high AMPS” product. Briefly, an aqueousmixture of polyvinyl alcohol macromer having acetal-linked ethylenicallyunsaturated groups and 2-acrylamido-2-methyl-propane sulphonate in aweight ratio of about 1:1 is suspended in a continuous phase of butylacetate containing cellulose acetate butyrate stabiliser with agitatorand is radically polymerised using redox initiation to form beads, whichare washed, dyed and sieved into size fractions including the 300-500μm, 500-700 μm and 700-900 μm fractions used in subsequent Examples. Theequilibrium water content of the microspheres is 94 to 95% by weight.

The loading procedure is shown schematically in FIG. 1. First, theaqueous packing solution of 1 ml beads was removed from the vial, andthe beads were fully mixed with 1 ml DMSO by gently shaking. After 5minutes the DMSO/water mixture was removed and 1 ml fresh DMSO was addedagain. The procedure was carried out three times, and a bead slurry wasfinally obtained by removing DMSO which also contained some removedwater. The water content at this stage is less than 10% based on theswollen weight assuming that DMSO equilibrates into the swollen beadsand based on the total volumes of water initially present as swolleninto the beads and DMSO added, taking into account the volume of mixedsolvents removed after each wash step.

16.4 mg paclitaxel (Taxol®) powder was dissolved in 1 ml DMSO in a vial.Subsequently, the prewashed water-depleted microsphere slurry was mixedwith the solution, and gently shaken for 10 minutes to enable the drugto completely diffuse into the beads. After this the DMSO solution wasremoved, 5 ml saline was added to the beads and the mixture wasagitated. Paclitaxel precipitates were observed in solution, and theblue beads became a white colour due to the precipitation of paclitaxelwithin the rehydrated beads as solvent changed. After removing thesolution and paclitaxel precipitates, 5 ml of fresh saline was added,and the procedure was repeated 5 times to remove paclitaxel crystals insolution and the residual DMSO.

The surface of the paclitaxel-loaded beads had attached to it many smalldrug crystals, and the paclitaxel domain inside the beads was notphysically stable (tending to recrystallise outside the beads). Inaddition paclitaxel has been reported as not chemically stable inaqueous solution. Therefore, the paclitaxel-loaded beads need to bedried.

The paclitaxel-loaded bead slurry was washed with 3 ml acetone for about1 minute, and the acetone was removed quickly by aspiration. Then thebeads were dried under a gentle stream of compressed air in an isolatorfor about 10 minutes. The beads were further dried by being left in afume hood overnight.

The paclitaxel loading was analysed by HPLC method. 4.1 mg ofpaclitaxel-loaded beads were extracted with 1 ml DMSO underultrasonication for 2 hr, subsequently extracted with fresh DMSO 1 ml 5times. The collected DMSO was analysed by Waters 486 HPLC system using aPhenomenex Luna C18 column (column temperature: 40° C.); mobile phase:methanol 63, ammonium acetate buffer (pH 3) 33, isopropanol 4 mixture;UV detector at 230 nm. The paclitaxel loading efficacy against targetloading (i.e. initial total amount of drug in solution) is shown in FIG.2.

The rehydration capability of the loaded beads was assessed by opticalmicroscopy comparing the size of the loaded beads before and afterrehydration. The volume increase of the rehydrated beads is contributedfrom the water uptake during bead swelling. In this case the drugelution amount was neglected. The dried beads with 7.2 mg/ml paclitaxelloading had an average diameter of 353±25 μm. After rehydration for 12h, the swollen beads had an average diameter of 810±46 μm. For the highloading beads with 23.9 mg/ml paclitaxel, the average diameter of drybeads is 399±20 μm. The average diameter after 12 h rehydration is832±45 μm.

FIG. 3 shows photos taken by using an Olympus microscope with aColorView camera. FIGS. 3A and 3C are the photographs of driedpaclitaxel-loaded beads, which show an irregular surface morphology.After rehydration, the beads swell back to spherical shape (FIGS. 3B and3D).

15.8 mg of dried paclitaxel-loaded beads were mixed with 200 ml PBSbuffer on a roller mixer under room temperature. Two test samples wereused, one loaded with 7.2 mg/ml paclitaxel and the other with 23.9 mg/mlpaclitaxel. At predetermined time interval, 100 ml solution was removedand fresh PBS was added. The elution was determined HPLC processing asfor FIG. 2. The profile is shown in FIG. 4.

EXAMPLE 2 Preparation of Rapamycin-Loaded Microspheres

In this Example the microspheres were 300-500 μm PVA beads (as used inExample 1). The loading procedure is shown in FIG. 5. First, the aqueouspacking solution of 1 ml beads was removed, and the beads were fullymixed with 1 ml DMSO by gently shaking. After 5 minutes the DMSO wasremoved and 1 ml fresh DMSO was added again. The procedure was repeatedthree times, and a bead slurry was finally obtained by removing DMSO.

Rapamycin powder (16.4 mg) was dissolved in 1 ml DMSO in a vial.Subsequently, the pre-washed water-depleted microspheres were mixed withthe rapamycin solution, and gently shaken for 10 minutes to enable thedrug to completely diffuse into the beads. After this the DMSO solutionwas removed, 5 ml of saline was added to the beads and the mixture wasagitated. Rapamycin precipitates were observed in solution, and the bluebeads became white in colour due to the solubility decrease of rapamycinwithin the rehydrated beads. After removing the solution and rapamycinprecipitates, 5 ml of fresh saline was added, and the procedure wasrepeated 5 times to remove rapamycin solid in solution and the residualDMSO.

The rapamycin loading was analysed by UV at 280 nm. 0.5 ml ofrapamycin-loaded beads was extracted with 2 ml DMSO 6 times. The actualloading and loading efficacy against target drug loading were show inFIG. 6. The UV analysis indicates that rapamycin loading efficacy is40±3% at different target loading.

FIG. 7 shows the photos taken by using an Olympus microscope with aColorView camera. FIG. 7A is the photograph of beads before rapamycinloading, which shows the transparent microspheres. After loading, thebeads were no longer transparent (FIG. 7B, 7C, 7D), but still kept theirspherical shape. FIG. 8 shows the histogram of size distribution ofbeads with/without rapamycin loading in saline. It demonstrates thatrapamycin loading has little effect on bead size distribution.

0.5 ml of rapamycin-bead slurry with different loading was mixed with200 ml PBS buffer on a roller mixer under room temperature. Atpredetermined time intervals, 100 ml solution was removed and fresh PBSwas added. The concentration of rapamycin in the PBS is determined by UVat 280 nm. The elution profile was shown in FIG. 9.

EXAMPLE 3 Preparation of Dexamethasone-Loaded Microspheres

Following the procedure in Example 2 and FIG. 5, dexamethasone wasloaded into PVA beads (300-500 μm) as used in Example 1. First, theaqueous packing solution of 1 ml beads was removed, and the beads werefully mixed with 1 ml DMSO by gently shaking. After 5 minutes the DMSOwas removed and 1 ml fresh DMSO was added again. The procedure wasrepeated three times, and a bead slurry was finally obtained by removingDMSO.

Dexamethasone powder (139.6 mg) was dissolved in 1 ml DMSO in a vial.The solubility of dexamethasone in DMSO at room temp is about 60 g/l,and it is practically insoluble in water. Subsequently, the pre-washedwater-depleted microspheres were mixed with the dexamethasone solution,and gently shaken for 10 minutes to enable the drug to completelydiffuse into the beads. After this the DMSO solution was removed, 5 mlsaline was added to the beads and the mixture was agitated.Dexamethasone precipitates were observed in solution, and the blue beadsbecame white colour due to the solubility decrease precipitation ofdexamethasone within the rehydrated beads as solvent changed. Afterremoving the solution and suspended dexamethasone precipitates, 5 ml offresh saline was added, and the procedure was repeated 5 times to cleardexamethasone solid in solution and the residual DMSO. Photographs ofthe dexamethasone-loaded beads under an optical microscope are shown inFIG. 10. There is a thin surface layer with substantially no drug. It isbelieved that the surface layer of drug is removed at the first step inwhich aqueous liquid is added (saline to precipitate the drug) since atthis stage there is sufficient solvent present in the liquid mixture tokeep the drug in solution at the surface so that it is removed with thesaline and further saline rinses.

EXAMPLE 4 Paclitaxel Loading by Different Solvent

Other water-miscible solvents, including 1-methyl-2-pyrrolidinone,N,N-dimethylformamide, and 1,4-dioxane were utilised in place of DMSO toload paclitaxel into beads (as used in Example 1). The procedures wereotherwise the same as that described in Example 1. Beads (500-700 μm)were used in this experiment. Before paclitaxel loading, the beads werewashed with the selected organic solvent and saturated while depletingthe water content values of below 15%. For each solvent adequatedilution is achieved with three washing steps. It was found that1-methyl-2-pyrrolidinone made the beads swell; N,N-dimethylformamide anddioxane decreased the bead sizes the size when swollen to equilibrium inPBS at room temperature. 1 ml of the organic solvent with dissolvedpaclitaxel powder of the concentration shown in the table below wasmixed with the beads above and left for 15 to 30 minutes. After salineprecipitation then washing and acetone rinsing, the beads were driedunder a gentle stream of compressed air, then in fume hood. The loadedpaclitaxel was extracted with DMSO and analysed by HPLC. The resultswere listed in the Table below.

Paclitaxel solution concentrations and loading efficiency in beads withorganic solvents Solvent Solution Conc^(n) g/l Loading efficiency1-methyl-2-pyrrolidinone 28.4 10.1% N,N-dimethylformamide 43.9 6.4%1,4-dioxane 31.1 0.8%

1. A process for forming drug-loaded polymer particles comprising thesteps: a) providing particles comprising a matrix of water-insolublepolymer, which, when neat, are swellable in phosphate buffered saline(PBS) at room temperature to an equilibrium water content in the rangeof from 40% to 99% by weight based on polymer plus PBS; b) providing asolution of a drug having a water solubility of less than 10 g/l at roomtemperature, in a first organic solvent is selected to be capable ofswelling neat particles; c) containing the particles with the solutionof drug whereby a solution of drug in solvent becomes impregnated intothe particles; d) separating drug solution which has not impregnated theparticles in step c) from the impregnated particles and recovering theimpregnated particles; e) contacting the impregnated particles withaqueous liquid whereby drug is precipitated in the core of theparticles; f) providing a volatile, second solvent in which theparticles are less swellable, relative to their swellability in water,and which is a solvent for the drug, wherein the drug solubility in thesecond solvent is at least 0.1 g/l; and g) rinsing the particlesproduced in step e) with the second solvent whereby drug on and close tothe surface of the particles is removed with the second solvent; and,optionally h) drying the drug-loaded polymer particles by vacuum, orfreeze drying, or air flow to remove the second solvent.
 2. A processaccording to claim 1, wherein the particles swell in step c) when thesolution of drug becomes impregnated into the particles.
 3. A processaccording to claim 2, wherein the particles are swollen to equilibriumwhen the solution of drug becomes impregnated into the particles. 4.(canceled)
 5. (canceled)
 6. (canceled)
 7. (canceled)
 8. (canceled)
 9. Aprocess according to claim 1, in which the first and second solvents arethe same.
 10. A process according to claim 1, in which the first andsecond solvents are different to one another.
 11. A process according toclaim 1, in which the first and/or second solvent is miscible withwater.
 12. A process according to claim 1, wherein the second solventhas a boiling point of less than 90° C.
 13. A process according to claim1, in which the ratio of drug solubilities at room temperature in thefirst organic solvent and water is in the range 10:1 to 10⁶:1.
 14. Aprocess according to claim 1, in which the drug is selected from thegroup consisting of rapamycin analogues, esters and salts thereof whichhave water solubility less than 0.1 g/l; paclitaxel; analogues, estersand salts thereof which have water solubility less than 0.1 g/l;dexamethasone and ibuprofen.
 15. A process according to claim 1, inwhich the first solvent is an aprotic solvent, from dimethyl sulfoxide(DMSO), 1-methyl-2-pyrrolidinone (NMP), dimethyl formamide (DMF) anddioxane.
 16. A process according to claim 3, in which the polymer matrixis formed from a polyvinyl alcohol macromer, having more than oneethylenically unsaturated pendant group per molecule, by radicalpolymerisation of the ethylenic groups.
 17. A process according to claim16, in which the macromer is copolymerised with ethylenicallyunsaturated monomers.
 18. A process according to claim 1, in which theparticles used in step a) have a mean particle size in the range 50 to1500 μm, when fully swollen with PBS at room temperature.
 19. Drugloaded polymer particles having a homogeneous polymer composition fromthe centre to the periphery, having precipitated drug in a core regionthereof, which drug has a water solubility of less than 0.1 g/l at roomtemperature; and having a surface layer in the range of from 1 to 100 μmthick, wherein the ratio of the concentration of drug in thecore:concentration of drug in the surface layer is at least 2:1. 20.Particles according to claim 19, in which the drug has a solubility in asolvent selected from dimethyl sulfoxide (DMSO),1-methyl-2-pyrrolidinone (NMP), dimethyl formamide (DMF) and dioxane toa concentration of at least 10¹, a times its solubility in water at roomtemperature.
 21. A process according to claim 15 in which the firstsolvent is selected from the group consisting of dimethyl sulfoxide(DMSO), 1-methyl-2-pyrrolidinone (NMP), dimethyl formamide (DMF) anddioxane.
 22. A process according to claim 1 in which the water insolublepolymer is a polyvinyl alcohol.
 23. A process according to claim 22 inwhich the water insoluble polymer is a covalently cross-linked polyvinylalcohol.
 24. A process according to claim 17 in which theethylenically-unsaturated monomer is selected from non-ionic monomers,and ionic monomers and mixtures thereof.
 25. A process according toclaim 17 in which the monomers include anionic monomers.
 26. A processaccording to claim 18 in which the mean particle size is in the range 50to 1500 μm.
 27. Drug loaded polymer particles according to claim 19 inwhich the said ratio is at least 10:1.
 28. Particles according to claim20 in which the said solubility of the drug in the solvent is at least102 times its solubility in water.